[RFC please help] membarrier: Rewrite sync_core_before_usermode()

[Nicholas Piggin]

Excerpts from Andy Lutomirski's message of December 29, 2020 10:36 am:
> On Mon, Dec 28, 2020 at 4:11 PM Nicholas Piggin <npiggin at gmail.com> wrote:
>>
>> Excerpts from Andy Lutomirski's message of December 28, 2020 4:28 am:
>> > The old sync_core_before_usermode() comments said that a non-icache-syncing
>> > return-to-usermode instruction is x86-specific and that all other
>> > architectures automatically notice cross-modified code on return to
>> > userspace.  Based on my general understanding of how CPUs work and based on
>> > my atttempt to read the ARM manual, this is not true at all.  In fact, x86
>> > seems to be a bit of an anomaly in the other direction: x86's IRET is
>> > unusually heavyweight for a return-to-usermode instruction.
>>
>> "sync_core_before_usermode" as I've said says nothing to arch, or to the
>> scheduler, or to membarrier.
> 
> Agreed.  My patch tries to fix this.  I agree that the name is bad and
> could be improved further.  We should define what
> membarrier(...SYNC_CORE) actually does and have arch hooks to make it
> happen.
> 
>> > So let's drop any pretense that we can have a generic way implementation
>> > behind membarrier's SYNC_CORE flush and require all architectures that opt
>> > in to supply their own.  This means x86, arm64, and powerpc for now.  Let's
>> > also rename the function from sync_core_before_usermode() to
>> > membarrier_sync_core_before_usermode() because the precise flushing details
>> > may very well be specific to membarrier, and even the concept of
>> > "sync_core" in the kernel is mostly an x86-ism.
>>
>> The concept of "sync_core" (x86: serializing instruction, powerpc: context
>> synchronizing instruction, etc) is not an x86-ism at all. x86 just wanted
>> to add a serializing instruction to generic code so it grew this nasty API,
>> but the concept applies broadly.
> 
> I mean that the mapping from the name "sync_core" to its semantics is
> x86 only.  The string "sync_core" appears in the kernel only in
> arch/x86, membarrier code, membarrier docs, and a single SGI driver
> that is x86-only.  Sure, the idea of serializing things is fairly
> generic, but exactly what operations serialize what, when things need
> serialization, etc is quite architecture specific.

Okay, well yes it's x86 only in name, I was more talking about the 
concept.

> Heck, on 486 you serialize the instruction stream with JMP.

x86-specific aside, I did think the semantics of a "serializing 
instruction" was reasonably well architected in x86. Sure it could do 
other things as well, but if you executed a serializing instruction,
then you had a decent set of guarantees (e.g., what you might want
for code modification).

> 
>> > +static inline void membarrier_sync_core_before_usermode(void)
>> > +{
>> > +     /*
>> > +      * XXX: I know basically nothing about powerpc cache management.
>> > +      * Is this correct?
>> > +      */
>> > +     isync();
>>
>> This is not about memory ordering or cache management, it's about
>> pipeline management. Powerpc's return to user mode serializes the
>> CPU (aka the hardware thread, _not_ the core; another wrongness of
>> the name, but AFAIKS the HW thread is what is required for
>> membarrier). So this is wrong, powerpc needs nothing here.
> 
> Fair enough.  I'm happy to defer to you on the powerpc details.  In
> any case, this just illustrates that we need feedback from a person
> who knows more about ARM64 than I do.
> 

Thanks,
Nick